1. Field of the Invention
The present invention relates to an annular optical member made of synthetic resin and, more precisely, it relates to an annular optical member which is provided on its inner surface with a bottomed inner groove.
2. Description of the Related Art
The cam ring of a zoom lens is usually provided on its peripheral surface with a cam groove in which an association pin of a movable lens group, which is movable in the optical axis direction, is fitted, so that the rotation of the cam ring causes the movable lens group to move in the optical axis direction and effect the zooming.
In a conventional cam ring made of metal, the cam groove is usually in the form of through grooves which extend completely through the cam ring. A cam ring made of synthetic resin has also been known to be used, in which the cam groove is a bottomed inner cam groove. Using molded products (e.g., cam rings made) of synthetic resin can reduce manufacturing costs because, unlike a through cam groove which must be covered by a separate light intercepting member provided on the outer surface thereof, the bottomed inner cam groove does not require such a separate light intercepting member. Furthermore, the bottomed cam groove does not significantly reduce the strength of the cam ring, in comparison with the use of a through cam groove.
A die for molding a cam ring having a bottomed cam groove has a projection corresponding to the bottomed cam groove. The projection of the molding die is made, as smoothly as possible, in accordance with the cam profile. To this end, the bottomed cam groove has smooth side (right and left) cam profile surfaces, as well as a smooth bottom surface. However, it has been found that the smooth bottom surface causes inner reflections within the lens.